Sunday, August 19, 2012

Some more notes on solar electrical

A friend recently asked some advice for setting up a solar electrical system for someone's van. Here's basically what I told him, from my experience.

A solar electrical system is a misnomer. It's not really a solar electrical system. It's a BATTERY electrical system. The battery is the single most important and crucial gating factor in the system, especially if you're going to be living in it full-time. It's all about the battery. This is because sunlight is ephemeral: it's there, then it's not, and you can't really predict how much you're going to get and when. It's not like you just plug into a grid and the power is always on. So you really are running on battery most of the time. Scrimp on battery and your system is going to be a mess. Get the battery right and everything will work fine, and even will tolerate mistakes.

I recommend only the best AGM batteries. Don't bother with flooded batts or even deep-cycle marine batts. Go for the good stuff: AGM's from the top manufacturers (when I bought mine, the best was Concord Lifeline, it may still be today, I don't know).

The next thing is: actually COUNT up your load! Do it in AMP/HOURS AT 12 VOLTS. It's crucial to keep the units consistent, and to use the units your battery capacity is measured in. My friend argued with me about this, but I'm going to stick to my guns. In a purely mathematical sense, he's right: you can use any units. But psychologically, and for simplicity, and to avoid errors, use the units your battery is specc'ed out in: amp/hours @ ~12v. So convert ALL your loads to amps @ 12v, and calculate the duty cycle-- how long you're going to be running it-- and make that amp/hours. Example: I run my netbook for 12 hours a day on average, and it consumes 1Amp, so it's burning 12A/h per day. For 120v devices, convert to 12v by multiplying the amps by 10. So 5A @ 120v is 50A @ 12V. That would be... a lot! See what I'm saying about psychologically? It's a larger number at 12v. This is important for realizing how much you're really drawing out of that battery.

Actually MEASURE the loads of your devices, don't go by the garbage that's on in the manual or printed on the back of the device or on its power brick. The UL listing on electrical items is for the maximum rating OF THE WIRE, not what it actually draws. So if you see 5A @ 120V, that means the wiring won't catch fire at 5A @ 120v. It doesn't mean the thing actually draws that! It might draw a tiny fraction of that. So put an ammeter or a kill-a-watt on the things and see what they draw (and then convert that to 12v).

Avoid 110V devices as much as possible. I have ZERO 110v devices running in my van on a daily basis. Once every few months (and only in bright sunlight!), I hook up my laser printer and turn on the inverter, for a few minutes. Everything else in here is converted to run on straight 12v. I have 12v-5v DC-DC converters (often called "chargers" or "adapters") for running things like phones and tablets. My refrigerator is 12v. I have a 12vDC shaver. My computers all have DC-DC converters that go straight to 12v. Lights are all 12v (and LED to boot, which is super low current).

Another thing is: you will overdesign your system by a huge margin, because you have to. Batteries can only use 50% of their rated capacity, and ideally you don't want to go less than maybe 75% of it. So if your battery says "130 Amp/hours", that's BS. It's more like 60Amp/hours, or really 50Amp/hours if you don't want to stress your battery too much. Same with solar panels: they only give their rated capacity in perfect full sunlight at noon. Most of the time they put out a LOT less. Like Flava Flav said: "don't believe the hype". Figure on a lot less.

Use only MPPT controllers. Don't bother with PWM controllers; they are not nearly as efficient, and you'll be wasting precious sunlight (plus PWMs are noisy and they give off more heat than MPPTs). I recomend the TrakStar SunSaver MPPT controller.-- quite possibly the best thing I ever bought for my van. It was the only affordable MPPT controller on the market at the time; I think I paid $150 for it used on eBay, but most MPPT controllers cost like $600 or thousands of dollars. So that's a good deal.

Also, batteries take TIME to charge and they can only absorb a certain amount of current at a time-- another reason to overdesign your battery capacity. High quality batteries will come with a data sheet showing the actual curves and giving the formulas. You can get away without all the math, perhaps, but do consider that they take time.

Also, your SunSaver MPPT will only put out 15A maximum in full sunlight no matter how many solar panels you've got. So that will limit how many solar panels you can practially actually use, and that in turn will also limit what you can draw at night.

And, if you have lots of battery capacity, and you make the mistake of deep discharging them (I did this, and found out the hard way), you'll never get them charged back up with just solar: you'll need a converter and you'll have to find some 110V place to plug in and charge back up in those situations. From there, the solar will keep the batteries topped up. So it's very good to have a 110V converter/charger around for those situations. And if you're going to be boondocking, and can fit one, a gasoline genset is a good backup to have.

Now, put all this together, and you realize: I really don't have a lot of power here to work with. It's true. Which brings us to the last thing:

WORK THE DEMAND SIDE! Try to limit how much load you have. Sell your MacBookPro and get a netbook: instead of drawing 4Amps you're drawing only 1Amp. Run your refrigerator on propane or get a super efficient Danforth-based 12v refrigerator. Use only LED lights. Don't run your fan if you don't have to.

The general process I recommend is an iterative one:
1) calculate your load 
2) calculate how much battery you'll need to run that load through a day or two without sun
3) decide how much solar you'll need to charge up the battery from that level of discharge
4) repeat and find ways to reduce your load (when you realize you'd need acres of solar panels and a thousand pounds of batteries to live as if you were in a stick house).

Thinking in terms of conserving electricity takes some getting used to. This is why I recommend calculating in terms of amp/hours instead of watts. TIME is an important factor! You only have so many hours of sunlight. You only have so many hours of battery. Amps-per-hour is the correct measure to use, not so much because it's any more mathematically correct than, say, watt/hours, but because it is psychologically good to get into thinking differently about electricity and how you use it.

No comments: